1. Field of the Invention
The present invention relates to an image forming process in which an electrostatic latent image formed by an electrophotographic process, an electrostatic recording process or a similar process is developed with a developer.
2. Description of the Related Art
Methods of making image data visible through an electrostatic latent image, such as electrophotography, are currently used in various fields. In electrophotography, an electrostatic latent image is formed on the surface of a photoreceptor through charging and exposing steps, and the latent image is developed with a developer containing a toner, followed by transferring and fixing steps to make the developed image visible.
The developers used for the development can be divided into two-component developers composed of a toner and a carrier, and single-component developers only made of a magnetic toner or a non-magnetic toner. In order to prepare toner particles for producing a developer, a kneading and pulverizing process is ordinarily used in which a thermoplastic resin is melted and kneaded with a pigment, a charging modifier and a releasing agent such as wax, and the resultant mixture is cooled, pulverized into fine particles and then classified. To the toner particles produced by the kneading and pulverizing process are added, at the surfaces thereof, inorganic or organic fine particles, as necessary, for improving fluidity and cleanability.
The shape of the toner particles produced by the conventional kneading and pulverizing process is irregular and the surface composition thereof is not uniform. The shape and the surface composition of the toner particles vary depending on pulverability of the materials used and conditions of a pulverizing step. It is difficult to control the shape and the surface composition of the toner particles. Particularly if a material that is easily pulverizable is used to produce the toner particles, there are problems in that the particles produced are too minute or have altered shapes owing to a mechanical force such as a shear force generated in a developing device.
When a two-component developer is used, the above described incidences cause the finely-pulverized toner particles to firmly adhere to a carrier, whereby chargeability of the developer is accelerated to deteriorate. When a single-component developer is used, the above-described incidences provide a broader distribution in particle size. Therefore, toners become likely to scatter or developability is lowered due to a change in the shape of the toner particles, whereby image quality is frequently deteriorated.
In a case where the shape of the toner particles is irregular, the fluidity of the toner is insufficient even if an auxiliary for fluidizing toners is added to the toner. Consequently, while the toner is used, there arise problems in that, owing to a mechanical force such as a shear force, minute particles of the auxiliary fall into cavities of the toner particles, whereby the fluidity of the toner decreases with the passage of time, and developability, transferability and cleanability are impaired. If such a toner is the toner collected once or twice after cleaning and then returned to a developing device to be re-used, the quality of the image formed with the toner is further deteriorated. In order to prevent such incidences from occurring, it is conceivable to increase the amount of the auxiliary to be added. In this case, however, problems arise in that black spots may appear on the photoreceptor and the auxiliary may be scattered.
If a releasing agent such as wax is added inside the toner particle, the releasing agent frequently appears at the surface of the produced toner particles depending on the combination of a releasing agent and a thermoplastic resin. Particularly in the case where a resin that has elasticity due to a high molecular weight component and hence is slightly difficult to pulverize is combined with a brittle wax such as polyethylene, polyethylene frequently appears at the surface of the produced toner. The thus produced toner has poor releasing performance at the time of fixing and poor cleanability of non-transferred toner present on a photoreceptor. However, polyethylene appearing at the particle surface is released from the surface by a shear force generated in a developing device. As a result, polyethylene is easily transferred to a developing roll, a photoreceptor, a carrier and the like. Contamination caused by transferring degrades reliability as a developer.
Under these circumstances, attempts have been made in recent years to solve the above-described problems by controlling the shape and the surface composition of toner particles, and methods of producing toners in a wet manner have been intensely studied. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-282749 and 6-250439 propose an emulsion polymerization aggregating process in which a dispersion of resin particles is prepared by emulsion polymerization while another dispersion is prepared in which a colorant is dispersed in an aqueous medium (solvent). Thereafter, the two solutions are mixed and heated to form aggregated particles whose particle size corresponds to a toner particle size, followed by further raising the temperature to effect coalescence of the aggregated particles, to finally produce a toner.
In recent years, high image quality has been increasingly demanded. Particularly in forming color images, trends are currently toward toner particles having a smaller and uniform size in order to achieve detail depiction. Generally, if a toner having a broad particle size distribution is used to form an image, toner particles belonging to a region of finer sizes in the distribution seriously cause contamination at a developing roll, a charging roll, a charging blade, a photoreceptor and a carrier, and also cause troublesome toner scattering. Accordingly, it is difficult to simultaneously achieve high image quality and high reliability. Further, the toner having a broad particle size distribution cannot yield high reliability in a system that has a cleaning function or a toner re-cycling function.
The toner having a finer particle size is likely to produce troubles particularly in a transferring step, to thus degrade high image quality. This is presumably attributed to the fact that an adhesive force of a toner to a photoreceptor, for example, a non-electrostatically adhesive force such as van der Waals force increases. In order to solve such a problem, it is necessary to adequately control an adhesive force between a toner and a photoreceptor, for example, by controlling the shape and the surface state of the toner particles.
As described above, it is very difficult to produce a toner having a very small and uniform particle size by employing a conventional kneading and pulverizing process. In principle, the smaller the particle size, the more the shape is distorted. Hence, it is impossible to avoid the above-described incidences from occurring in a transferring step. From the foregoing, an emulsion polymerization aggregating process has intensely been studied among wet toner-producing processes, since the process makes it possible to readily produce a toner having a very small and uniform particle size.
However, if toner particles are produced by employing an emulsion polymerization aggregating process, reaction usually progresses toward a goal, by applying heat, to make toner particles having irregular shapes to have a more smooth and spherical shape, that is, to make the particles to have a smaller surface area. Therefore, in principle, as the toner particles have a smaller particle size, the spherical degree thereof is higher (the surface area thereof is smaller); while as the toner particles have a larger particle size, the irregular degree of the shape is higher (the surface area thereof is larger). Moreover, in order to achieve all performances including transferability, transferring efficiency, cleanability and durability required of a toner, it is necessary to design the toner optimally.
For the above-described purpose, for example, JP-A No. 61-279864 proposes a potato-shaped toner in which toner shape coefficients SF1 and SF2 are specified within a range of 120 to 180 and within a range of 110 to 130, respectively, in which SF1 (an index representing toner distortion={[(maximum length)2/(projected area)]xc3x97(xcfx80/4)xc3x97100} and SF2 (an index representing surface roughness)={[(circumferential length)2/(projected area)]xc3x97(xc2xcxcfx80)xc3x97100}. However, when the thus obtained toner is used for oil-less fixing, a problem associated with fixing stability arises, whereby offset phenomena occasionally arise when a large number of sheets are copied.
The particle size distribution of a toner may mainly be affected by rupture of the toner by a mechanical force. Further, if a toner has an initial broad particle size distribution, granular selectivity during development, scattering during transfer and cleanability are adversely affected. In addition, if a toner has a broad particle size distribution, contamination frequently occur at a developing roll, a charging roll and a charging blade during development when a single-component developer is used.
In order to control variation in the shape of toner particles, it is necessary to narrow a distribution at both sides of a small particle size region and a large particle size region. Toner particles can be compared with each other with respect to a particle size distribution, using indexes designated as GSD each for a volume average and a number average. The volume average GSD (GSDv) and the number average GSD (GSDp) can be used as an index representing a proportion at a large particle size and as an index representing a proportion at a small particle size, respectively. It is particularly preferable to use GSDp-under as an index representing a proportion at a small particle size.
In view of developability and/or transferability, toner particles belonging to a small particle size have a strong adhesive force, as have been already known. Therefore, it is difficult to electrostatically control such particles, and such particles are likely to remain on a carrier when a two-component developer is used. If a mechanical force is repeatedly applied, carrier contamination is caused to promote carrier deterioration. Since toner particles having a small particle size have a strong adhesive force, developing efficiency decreases thus leading to defective image quality. In transferring step, among the whole toner particles developed, the particles belonging to a smaller size region are difficult in being transferred onto a photoreceptor. As a result, transferring efficiency decreases, whereby the amount of toner waste increases and image quality degrades.
On the other hand, the toner particles belonging to a large particle size region, although having a weak adhesive force, are likely to produce an uneven gap in transferring step or easily being scattered to a non-image area. Thus, the particles are largely associated with a decrease in image quality. Furthermore, as the particles are likely to scatter at the time of development, a decrease in reliability due to contamination inside the device is liable to occur. In a case where the toner shape is made close to a sphere to achieve high transferring efficiency, the aforementioned incidences become even more pronounced.
Toners undergo various stresses in the process of electrophotography. In order to obtain stable toner performances, it is necessary to control not only the shape and the particle size of a toner but also to hinder a releasing agent from appearing at the surface of the toner.
In general, when a kneading and pulverizing process is employed, an exposure ratio of the releasing agent at the toner surface is as high as 40%. Although such a high ratio favorably affects fixing performances such as prevention of offset phenomena from occurring at high temperatures, it is difficult to maintain charging properties over a long period of time, or it is apt to undesirably produce a black spot by adhesion of the releasing agent to a carrier or a photoreceptor. If the ratio is small, there arise problems mainly associated with decreased reliability of fixing performances, such as occurrence of offset phenomena at high temperatures or the lowered fixed image strength at low temperatures.
Usually, in order to fix to a receiving body an unfixed toner image which has been formed using the above-described toner, a heat-fixing method is widely used, in which a toner is melted by heating to cause thermal fusion.
As the heat-fixing method, heat-fixing is widely employed in which a recording medium having formed thereon an unfixed toner image is passed through two rollers, one being a heat-fixing roller having a built-in heating source and the other being a pressure roller capable of pressing a passing material against a counter roller, to thereby fuse and fix the toner onto the recording medium.
In the above-described heat-fixing, however, the heat-fixing roller must be coated with an elastic layer having a relatively large thickness or the elastic layer must be coated with a releasing layer in order to obtain high image quality or high fixing performances. Since it takes a long time for the heat-fixing roller to warm up, heat-fixing is not preferable from the viewpoint of saving energy.
The present invention was devised to overcome the above-described problems of prior art toners.
1. An object of the invention is to provide an image forming process which has excellent properties such as chargeability, developability, transferability, fixing properties, and particularly cleanability, satisfies high image quality, high reliability and sufficient maintaining ability, and makes it possible to stably perform instant-on fixing.
2. Another object of the invention is to provide an image forming process with which it is possible to produce a high quality image having excellent light transmission and colorability.
The means to attain the above-described objects are as follows.
A first aspect of the invention is an image forming process comprising the steps of: forming a latent image on a surface of an electrostatic latent image-bearing body; developing the latent image with a toner for developing an electrostatic latent image to form a toner image; transferring the toner image onto a receiving body; and fixing the toner image to the receiving body, wherein the step of fixing is carried out using a fixing device comprising a heat-fixing roller in which a heat-resistant elastic layer is provided on a cylindrical core metal and the resultant surface thereof is further provided with a heat-resistant resin layer, an endless belt, and a pressure member arranged inside the endless belt to allow the endless belt to travel around the heat-fixing roller at a given angle such that a nip is produced through which a recording sheet (the receiving body) passes, with the pressure member being pressed via the endless belt against the heat-fixing roller at the nip to thereby distort the heat-resistant elastic layer in the heat-fixing roller, and the toner for developing the electrostatic latent image satisfies the requirements of: a) a shape coefficient SF1 ranges from 125 to 140 and a shape coefficient SF2 ranges from 105 to 130, wherein SF1=(xcfx80/4)xc3x97(L2/A)xc3x97100 and SF2=(xc2xcxcfx80)xc3x97(I2/A)xc3x97100, in which L represents a maximum length, I represents a circumferential length and A represents a projected area of toner particles; b) an exposure ratio of a releasing agent at the surface of the toner particles determined by X-ray photoelectron spectroscopy (XPS) ranges from 11 to 40%; and c) a melting point of the releasing agent contained at 8 to 20% by mass in the toner, measured with a differential scanning calorimeter, ranges from 70 to 130xc2x0 C., and the size of the releasing agent determined at a cross section of the toner particle observed with a transmission electron microscope ranges from 150 to 1500 nm.
That is, by providing the heat-resistant elastic layer of the heat-fixing roller with the heat-resistant resin layer as the releasing layer and distorting the heat-fixing roller, high image quality can be achieved without using a releasing agent such as silicone oil while maintaining a high releasing property. The releasing property of the heat-resistant resin does not deteriorate easily, and it is possible to maintain the releasing property over a long period of time. Since the endless belt is wound around the heat-fixing roller to produce a nip, a larger nip can be obtained with merely a low load applied as compared to a nip obtained in a fixing method using a pair of rollers. Thus, stiffness of the core metal of the fixing roller can be decreased, and additionally the heat-resistant elastic layer of the heat-fixing roller may be thinned. Thus, instant-start ability can be improved. Since a load applied to a nip can be reduced, the wear of a heat-resistant resin layer can be considerably decreased.
If the shape coefficient SF1 of the toner exceeds 140, the fluidity of the toner decreases, whereby transferability of the toner is adversely affected from an initial stage. If the shape coefficient SF1 is below 125, insufficient cleaning may occur to cause contamination of the apparatus or a decrease in reliability. If the shape coefficient SF2 is below 105, insufficient cleaning may occur, which easily causes contamination of the apparatus or a decrease in reliability. If the shape coefficient SF2 exceeds 130, the fluidity of the toner is likely to decrease.
The shape coefficient of the toner is obtained as follows. The toner is sprayed onto a slide glass, and then an image is observed with an optical microscope and information of the image is sent through a video camera to a Luzex image analyzer. The maximum length and the projected area of each of over 1,000 toner particles are measured, and the obtained values are substituted for the variables in the above-described equations. The average values thereof are used as the shape coefficient.
If the ratio of the releasing agent appearing (exposed) at the toner surface is below 11%, the fixing performance is not affected at an initial stage, however, the maintaining ability may occasionally be affected over a long period of time. If the fixing device deteriorates, offset phenomena at high temperatures and fixed-image strength at low temperatures may sometimes be affected. On the other hand, if the ratio is over 40%, the fixing performance is not affected but filming may occur in the carrier, a developing roller and a photoreceptor. Moreover, such a phenomenon easily occurs where an externally added additive for imparting fluidity to the toner penetrates the toner. The surface exposure ratio can be determined, using a measuring instrument such as an X-ray photoelectron spectrometer (XPS) manufactured by JEOL. Ltd. which is capable of distinguishing the peaks obtained from the resin, the pigment and the wax.
The addition amount of the above-described releasing agent preferably ranges from 8 to 20% by mass. If the amount is smaller than this range, sufficient releasing property cannot be obtained, leading to insufficient releasing. If the amount is larger than the above-described range, light transmission after fixing the toner to the resin sheet is affected, thus impairing resin sheet adaptability and initial chargeability of the toner.
Furthermore, the size of the releasing agent at a cross section of the toner particles observed with a transmission electron microscope preferably ranges from 150 to 1500 nm. If the size of the releasing agent is smaller than this range, the releasing agent does not act well, thus failing to produce a sufficient releasing property. If the size is larger than the above-described range, light transmission after fixing the toner to the resin sheet is affected, thus impairing resin sheet adaptability.
The releasing agent preferably has a melting point, whose main maximum peak measured according to ASTMD 3418-8 lies within a range of 70 to 130xc2x0 C. If the peak occurs below 70xc2x0 C., offset phenomena easily take place at the time of fixing the toner. If the peak occurs over 130xc2x0 C., fixing temperature becomes high so that the surface of the fixed image cannot become smooth, to thus impair gloss. The main maximum peak is measured using, for example, DSC-7 manufactured by PerkinElmer Japan Co., Ltd. For temperature correction at a temperature sensing portion of the device, the melting points of indium and zinc are used. For calorie correction, the heat of fusion of indium is used. Measurements are conducted using a pan made of aluminum for a sample while a blank pan for a control, at a temperature-elevating rate of to 10xc2x0 C./minute.
According to the present invention having a characteristic feature of combining the above-described fixing device and the specified toner to form an image, instant-on fixing can be satisfactorily achieved and additionally high image quality can be maintained on a large number of sheets. Furthermore, according to the invention, an image excellent in light transmission and colorability can be obtained.
A second aspect of the invention is an image forming process in which the thickness of a heat-resistant elastic layer ranges from 0.2 mm to 1.0 mm.
In the invention, the thickness of a heat-resistant elastic layer preferably ranges from 0.2 mm to 1.0 mm. When the elastic layer having a lower heat conductivity than metals is thick, conduction of heat is slow even if the inside of the layer is heated. Therefore, a thick layer may prevent the high-speed operation of a fixing device. In the invention, the duration for the fixing device to be heated to 180xc2x0 C. is desirably 60 seconds or less. If the duration for the temperature of the device to reach 180xc2x0 C. is over 60 seconds, instant-start ability is poor and advantages of the toner used in the invention cannot sufficiently provided.
In the invention, the heat-resistant resin layer is desirably made of a fluorine-containing resin. The fluorine-containing resin has an excellent releasing property which does not easily deteriorate with the passing of time. Hence, the fluorine-containing resin renders the life span of the fixing device longer. Examples of the fluorine-containing resin include polytetrafluoroethylene (hereinafter referred to as xe2x80x9cPTFExe2x80x9d), perfluoroalkyl vinyl ether copolymer (hereinafter referred to as xe2x80x9cPFAxe2x80x9d), tetrafluoroethylene hexafluoropropylene copolymer (hereinafter referred to as xe2x80x9cFEPxe2x80x9d).
In the invention, the thickness of a heat-resistant resin layer in the heat fixing roller preferably ranges from 10 to 50 xcexcm. By making the heat-resistant resin layer thin, the heat-resistant elastic layer is effectively distorted at the nip, to thus improve the releasing property.
A third aspect of the invention is an image forming process in which the distortion is expressed by the magnitude of a nip width of 3 to 12 mm of the elastic layer in the heat-fixing roller.
In the invention, a pressure pad is used as the pressure member, and a nip width between a heat fixing roller and an endless belt, which is produced with a pressure pad, desirably ranges from 3 to 12 mm. If a pressure pad is used as the pressure member, the device can be made small-sized. If the nip width is smaller than the above-described range, sufficient heat and pressure cannot be applied to the receiving body so that sufficient fixing performance may not be obtained. If the nip width is larger than the above-described range, the endurance of the heat fixing roller or the traveling ability of the receiving body is undesirably impaired.
In the invention, it is preferable that a nip pressure generated when the pressure pad presses the heat-fixing roller is locally increased in the vicinity of the outlet of a nip. If the distortion of the fixing roller is enlarged locally in the vicinity of the outlet of a nip, a higher releasing property can be obtained with a smaller distortion as compared with the case of generating distortion over an entire nip as obtained by a fixing method using a pair of rollers. Accordingly, wrinkles can be prevented from being generated even when a thin heat-resistant resin layer is formed on the surface of a heat-fixing roller. Further peeling of the heat-resistant elastic layer from the releasing layer made of the heat-resistant resin does not easily occur, to thereby maintain a high releasing performance and reliability over a long period of time. Moreover, since the distortion magnitude is small, the heat-resistant elastic layer in the fixing roller can be thinned. Since this fact contributes to reduce the heat capacity of a fixing roller, instant-start ability can be further improved and power consumption can also be reduced. Since the heat-resistant elastic layer having a low thermal conductivity can be made thin, heat resistance between an inner face and an outer face of the fixing roller can be decreased so that the heat response may be facilitated. Thus, a swift fixation can be attained. Since the distortion magnitude is small, the wear of a heat-resistant resin can be reduced.
In the invention, the total pressing force exerted by the pressure pad is desirably 60 kg or less. If the pressing force is high, wrinkles may be generated when a thin heat-resistant resin layer is provided on the surface of the heat-fixing roller. As a result, the endurance of the heat-fixing roller significantly decreases. Further, the heat-resistant elastic layer may undesirably be peeled from the releasing layer made of the heat-resistant resin and hence the heat-resistant resin may easily be worn. Thus, the releasing performance cannot be maintained over a long period of time, to thereby degrade reliability. Moreover, the heat-resistant elastic layer of the fixing roller cannot be thinned, failing to readily achieve instant-start ability.
A fourth aspect of the invention is an image forming process in which the toner further satisfies the requirements of: d) an average volume particle size distribution index GSDvxe2x89xa61.25, wherein GSDv=(D84v/D16v)xc2xd, in which D84v is a particle size value at which accumulated volume from the side of a smaller particle size in the volume particle size distribution accounts for 84% and D16v is a particle size value at which accumulated volume from the side of a smaller particle size in the volume particle size distribution accounts for 16%; e) an average number particle size distribution index GSDpxe2x89xa61.25, wherein GSDp=(D84p/D16p)xc2xd, in which D84p is a particle size value at which accumulated number from the side of a smaller particle size in the number particle size distribution accounts for 84% and D16p is a particle size value at which accumulated number from the side of a smaller particle size in the number particle size distribution accounts for 16%; f) a small particle size side number particle size distribution index GSDp-underxe2x89xa61.27, wherein GSDp-under=(D50p/D16p), in which D50p is a particle size value at which accumulated number from the side of a smaller particle size in the number particle size distribution accounts for 50% and D16p is a particle size value at which accumulated number from the side of a smaller particle size in the number particle size distribution accounts for 16%; and g) inclusion of minute particles made of two or more kinds of silicon compounds, each having a central particle size of 5 to 30 nm and 30 to 100 nm, at 0.5 to 10% by mass.
If the average volume particle size distribution index GSDv is more than 1.25, both sharpness and resolution of images deteriorate. If the average number particle size distribution index GSDp is more than 1.25, transferability decreases from an initial stage to thereby degrade image quality. Specifically, if the small particle size side number particle size distribution index GSDp-under is more than 1.27, the proportion of small particle size toner particles becomes high, whereby the initial performance and reliability are significantly affected. In other words, as has been already known, since toner particles having a small size have a strong adhesive force, it is difficult to electrostatically control such particles, whereby such particles are likely to remain on a carrier if a two-component developer is used. When a mechanical force is repeatedly applied to the toners, carrier contamination frequently occurs to promote carrier deterioration. Since the small particle size toner particles have a strong adhesive force, developing efficiency also decreases, thus leading to defective images. In the transferring step, it is difficult to transfer small particle size particles, among the toner particles developed, onto a photoreceptor, resulting in a decrease in transferability, whereby the amount of toner waste increases and image quality degrades.
If the average volume particle size D50v is less than 3 xcexcm, cleanability becomes insufficient to thereby impair developability. If the size D50v is more than 7 xcexcm, the resolution of images easily deteriorates.
Examples of the minute particles of silicon compound include silica, hydrophobic silica, colloidal silica, cation surface-treated colloidal silica and anion surface-treated colloidal silica. These inorganic minute particles have undergone dispersing treatment beforehand in the presence of an ionic surfactant using an ultrasonic dispersing machine. Colloidal silica is preferably used since it does not need to undergo such a dispersing treatment.
If the addition amount of the silicon compound minute particles is less than 0.5% by mass, a sufficient toughness cannot be obtained when the toner is melted. Thus, the releasing property in an oil-less fixation cannot be improved and oil-less releasing property may be damaged since coarse dispersibility of the minute particles in the toner causes increased viscosity when the toner is melted, to thus impair spinning ability. If the addition amount is more than 10% by mass, a sufficient toughness can be obtained but the fluidity of the toner when melted decreases significantly to thereby impair gloss of images.
In the invention, the process preferably comprises steps of admixing a dispersion containing at least resin fine particles having a particle size of at least 1 xcexcm or less, with a dispersion of colorant particles, a dispersion of a releasing agent and a dispersion of inorganic minute particles, to prepare a dispersion of aggregated particles of resin fine particles and colorant particles, and thereafter heating the thus prepared dispersion to a temperature above the glass transition point of the resin fine particles to cause fusion or coalescence. In the aggregating step, at least one polymerized metal salt is used.
A fifth aspect of the invention is an image forming process in which the developing step is performed using a developer for developing an electrostatic latent image comprising a carrier and a toner for developing an electrostatic latent image.
A sixth aspect of the invention is an image forming process in which a toner is produced by a process comprising steps of admixing a dispersion containing at least resin fine particles having a particle size of 1 xcexcm or less, with a dispersion of colorant particles and a dispersion of a releasing agent to prepare a dispersion of aggregated particles, and thereafter heating the thus prepared dispersion to a temperature above the glass transition point of the resin fine particles.